This invention relates to the field of optimizing tire uniformity, and more particularly to a method of correcting or shifting the conicity value, in combination with improving radial run-out, radial force variation, and lateral force variation characteristics of a pneumatic tire by grinding the tread surface in order to improve automobile directional stability.
In the art of manufacturing pneumatic tires, rubber flow in the mold or minor differences in the dimensions of the belts, beads, liners, treads, plies of rubberized cords, sometimes cause non-uniformities in the final tire. When non-uniformities are of sufficient magnitude, they will cause force variations on a surface, such as a road, against which the tires roll and thereby produce vibrational and acoustical disturbances in the vehicle upon which the tires are mounted. Regardless of the cause of the force variations, when such variations exceed the acceptable minimum level, the ride of a vehicle utilizing such tires will be adversely affected.
The effects of non-uniformity are best explained by noting that several types of forces, which are of particular importance in the present application, are simultaneously exerted by a tire during its rotation under load against a surface. For example, radial run-out is an intrinsic tire non-uniformity best described as the xe2x80x9cout of roundnessxe2x80x9d of the tire. Also radial forces are exerted in the radial direction of the tire, or in a direction perpendicular to its axis of rotation and non-tangential to the road surface. Additionally, lateral forces are exerted in the axial direction of the tire or in a direction parallel to its axis of rotation. Further, excessive conicity, defined as one-half of the net average lateral force resulting from a non-conical shaped tire, causes a tire to constantly pull to one side.
In a non-uniform tire, the radial run-out, the radial forces, and the lateral forces exerted by the tire will vary or change during its rotation. In other words, the magnitude and/or direction of the radial run-out, and the radial and lateral forces exerted by the tire will depend on which increment of its tread is contacting the surface.
The variations in radial and lateral force during rotation of a tire are usually caused by differences in the stiffness and/or geometry of the tire about its circumference or tread centerline. If these differences are slight, the radial and lateral force variations and therefore the degree of conicity will be insignificant and their effects unnoticeable when the tire is installed on a vehicle. However, when these differences reach a certain level, the radial and/or lateral force variations may be significant enough to cause rough riding conditions and/or difficult handling situations. Also, an excessive conicity value will cause a rolling tire to pull to one side.
Consequently, methods have been developed in the past to correct for excessive force variations by removing rubber from the shoulders and/or the central region of the tire tread by means such as grinding. Most of these correction methods include the steps of indexing the tire tread into a series of circumferential increments and obtaining a series of force measurements representative of the force exerted by the tire as these increments contact a surface. This data is then interpreted and rubber is removed from the tire tread in a pattern related to this interpretation. These methods are commonly performed with a force variation machine which includes an assembly for rotating a test tire against the surface of a freely rotating loading drum. This arrangement results in the loading drum being moved in a manner dependent on the forces exerted by the rotating tire whereby forces may be measured by appropriately placed measuring devices. In a sophisticated force variation machine (FVM), such as a Model No. D70LTW available from the Akron Standard Co. of Akron Ohio. The force measurements are interpreted by a computer and rubber is removed from the tire tread by grinders controlled by the computer. Examples of these methods are disclosed for example in U.S. Pat. Nos. 3,739,533, 3,946,527, 4,914,869, and 5,263,284.
As illustrated by prior patents and commercial devices, as described above, efforts are continuously being made to more efficiently correct tire non-uniformity. None of these prior art efforts, however, suggest the present inventive combination of method steps and component elements arranged and configured for correcting the conicity parameter, as well as the order of the routine for correcting variations in lateral forces, followed by radial run-out and finally radial forces as disclosed and claimed herein. Prior methods and apparatus do not provide the benefits of the present invention which achieves its intended purposes, objectives and advantages over the prior art devices through a new, useful and unobvious combination of method steps and component elements, through no increase in the number of functioning parts, at a reduction in operational cost, and through the utilization of only readily available materials and conventional components.
It is an object of the present invention to provide a method for correcting or shifting the conicity value in combination with improving radial run-out, radial force variation, and lateral force variation characteristics of a pneumatic tire to obviate the problems and limitations of the prior art methods. Other objects of this invention will be apparent from the following description and claims.
In accordance with the invention, there is provided a method for correcting or shifting the conicity value of a tire. To determine the conicity, the average lateral force exerted on the load wheel of a force variation machine by the tire turning in both the clockwise and counterclockwise direction is determined and analyzed with a computer program. The computer program then checks or scans a number of preset variables to determine if the conicity needs to be altered. These preset variables include the type of tire, the rate of conicity change for a specific type of tire, and the amount of power used by the motor turning the grinding wheel. The computer then calculates the difference between the actual value of the measured conicity and a specified conicity range or target conicity value. If the actual value of the conicity is within a first specified range, the conicity grind is discontinued and additional corrective grinding procedures are initiated. If the actual conicity is outside of the first specified range but within a second specified range extending above and below the first specified range, the tire is ground to a conicity value within the first specified range. Finally, if the actual conicity is outside of the second specified range, the tire is discarded. While the tire can be ground to within a specified range, it can also be ground to a specific conicity value.
In the case where the tire is to be ground, the computer determines the direction of conicity shift desired and the proper grinder with which to perform the grind. The computer next calculates the amount of time to grind and signals the selected grinder to move into position against the tire. While the selected grinder grinds the surface at a specified power for the determined length of time, the power used by the selected grinder is kept at the specified power level. After the selected grinder has engaged the tire for a determined length of time, the grinder is moved away from the tire and the computer routine is rerun to test the tire and determine the conicity value after the grind. If the computer determines that a further conicity shift is required, the program repeats and another grind is performed.
Further according to the invention, subsequent to the conicity grind, the tire is subjected to three consecutive, corrective grind routines; the lateral force variation corrective grind; the radial run-out corrective grind; and finally, the radial force variation corrective grind. The order of these corrective grinds is an important feature of the present invention.
In accordance with one embodiment of the invention, the grind routine can be used to control a center grinder apparatus to grind a central region of the tire tread between the shoulders of the tire tread to correct for conicity.
In accordance with another embodiment of the invention, an alternative conicity grind routine is used in combination with a force variation machine with shoulder grinders. The grind routine utilizes a feedback control loop to grind the tires to a desired value of counter-clockwise lateral shift. The computer controls one of the shoulder grinders to grind a shoulder of the tire until the value of the average lateral force in the counter-clockwise direction is above or below a calculated target counter-clockwise lateral shift value. Since the construction of the force variation machine requires that all grinding occur only when the tire is rotating in the counter-clockwise direction, the counter-clockwise lateral shift is measured and adjusted to shift the conicity value during grinding of the tire. The grind routine can incorporate simultaneous grinding and measuring of radial run-out, radial force variation, and/or lateral force variation characteristics of the tire.